This invention relates to aromatic polyimide films coated with a thermally stable, heat-sealable thermoplastic polyimide polymer of composition defined hereinbelow. The coated film is useful as a wire insulation for superconducting magnets.
Particle accelerators are presently in use which operate at low energies and employ magnets which are relatively small and are subject to low levels of stress in operation. Such conventional magnets are insulated with a polyimide film bonded with an epoxy impregnated glass cloth.
More recently, high energy accelerators are being designed using superconducting (SSC) magnets employing superconducting wire. The magnets are larger and operate at much higher energies producing very high stresses. The magnets are prestressed when they are fabricated to a higher stress than they will see in operation to eliminate small movements which would interfere with the stable operation of an accelerator. Conventional epoxy impregnated glass cloth binder exhibits brittleness at the extreme temperatures required for SSC magnets and such binder lacks adhesion to polyimide insulation. Further, stress-cracks initiate frequently in the cured epoxy and continue to propagate completely through to the polyimide to the bare superconducting wire.
Polyimide film is a preferred insulation for high energy magnets due to its superior resistance to ionizing radiation. However, polyimide film has been found to cold-flow at the high prestresses used in new accelerator designs. Such cold-flow results in a premature loss of prestress leading to quenching of the magnets often with catastrophic results.
U.S. Pat. No. 3,505,168, issued to Dunphy et al. on Apr. 7, 1970, discloses a heat-sealable laminar structure having at least two layers of polyimide, wherein the polyimide of one layer is different from the polyimide of the other layer.
U.S. Pat No. 3,616,177, issued to Gumerman on Oct. 26, 1971, discloses a laminar article having a layer of polyimide and a layer of a fluorinated polymer for use as a tape for electrical insulation.
U.S. Pat. No. 3,770,573, issued to Dunphy et al. on Nov. 6, 1973, discloses laminar film structures having one non-heat-sealable polyimide layer interdiffusionally bonded to a heat-sealable layer for use in the form of narrow tapes as an insulation wrap for electrical conductors.
None of these patents provides a suitable teaching without gross deficiencies for formulating a suitable insulating film for wrapping a superconducting conductor.
It has now been found that a coated aromatic polyimide film containing inorganic particles minimizes cold-flow (i.e., creep) when used as a primary insulation for SSC magnets. Moreover, the polyimide film when coated on one or both sides with a thermoplastic polyimide of the type defined hereinbelow effects sufficient adhesion between the superconducting stacks of wire needed for magnet assembly with sufficient high radiation resistance to minimize polymer degradation during the life of the magnet.